A multiple-ratio automatic transmission in an automotive vehicle powertrain utilizes multiple friction elements for automatic gear ratio shifting. In general, these friction elements may be described as torque establishing elements although more commonly they are referred to as clutches or brakes. The friction elements establish power flow paths from a torque source such as an internal combustion engine or a traction motor to vehicle traction wheels. During acceleration of the vehicle, the overall speed ratio, which is the ratio of a transmission input shaft speed to a transmission output shaft speed, is reduced as vehicle speed increases for a given accelerator pedal demand as the transmission upshifts through the various ratios.
In the case of a synchronous upshift, a first torque establishing element, referred to as an off-going clutch (OGC), is released while a second torque establishing element, referred to as an on-coming clutch (OCC), is engaged to lower a transmission gear ratio and change the torque flow path through the transmission. A typical upshift event is divided into a preparatory phase, a torque phase, and an inertia phase. During the preparatory phase, the OCC is stroked to prepare for its engagement while the OGC torque-holding capacity is reduced as a step toward its release. During the torque phase, which may be referred to as a torque transfer phase, the OGC torque is reduced toward a value of zero or a non-significant level to prepare it for disengagement. Simultaneously, the OCC torque is raised from a non-significant level, thereby initiating engagement of the OCC according to a conventional upshift control strategy. The timing of the OCC engagement and the OGC disengagement results in a momentary activation of two torque flow paths through the gearing, thereby causing torque delivery to drop momentarily at the transmission output shaft. This condition, which can be referred to as a “torque hole,” occurs before disengagement of the OGC. A vehicle occupant can perceive a “torque hole” as an unpleasant shift shock. When the OCC develops enough torque, the OGC is released, marking the end of the torque phase and the beginning of the inertia phase. During the inertia phase, the OCC torque is adjusted to reduce its slip speed toward zero. When the OCC slip speed reaches zero, the shift event is completed.
Torque hole filling is the process by which the transmission control strategy attempts to reduce and/or eliminate the transmission output torque hole during an upshift event. Control strategies for reducing torque disturbances include providing an increase in transmission input torque during the torque phase of the upshift. The increase in transmission input torque must be synchronized with the OCC and OGC to deliver a consistent shift feel. Various techniques and/or strategies may be used to increase transmission input torque, such as, throttle and spark timing control of the engine. The throttle may be opened more than required to achieve driver demand torque and the spark may be retarded to maintain the same engine torque. This strategy creates a torque reserve where the engine can quickly provide more transmission input torque. However, there are various limitations associated with use of this approach; for example, external conditions (e.g., high altitude) may prevent the engine from creating the desired torque reserve, which would reduce the overall effectiveness of the torque hole filling strategy. As such, there is a need to provide a robust and systematic means to reduce torque disturbances transmitted from the powertrain to the vehicle body during an upshift event.